1. Field of the Invention
The present invention relates to a shadow mask and a color CRT, and more particularly to a shadow mask having a structure in which a shadow mask is provided in only one direction, and to a color CRT that uses a shadow mask of this construction.
2. Background of the Invention
We will show two references as follows.
The cited reference 1 denotes a color selecting mechanism of cathode-ray tube and frame for its color selecting mechanism which is disclosed in Japanese Unexamined Patent Application Publication No. 10106449A. This application intends to obtain a curved surface where a frame surface is single or close to this by constituting so that a surface to which a color selecting electrode thin plate is fixed is formed as a curved surface having at least one or more inflection points while proceeding to an end part from a central part.
After support members of a frame are molded by press working or the like so that the whole become respectively necessary curvature in the Y direction and the Z direction, a curved surface of a surface to which the support members are welded is formed by cutting work. The surface is a curved surface on/to which a color selecting electrode thin plate is stretched/welded, and is defined by a fifth-degree equation expressed by, for example, an equation. In this way, since the surface to which the color selecting electrode thin plate of the frame is fixed is formed as a curved surface having at least one or more inflection points while proceeding to an end part of a central part, a curved surface where a frame surface is single or close to this, can be obtained.
The cited reference 2 denotes a cathode-ray tube which is disclosed In Japanese Unexamined Patent Application Publication No.11273586A. This application intends to prevent deterioration of images due to vibration of an AG tape.
In this cathode-ray tube comprising an aperture grill having a large number of slit holes, a frame supporting the aperture grill, and a color selecting mechanism having vibration control wires tightly stretched on the aperture grill, the aperture grill are supported by the frame so as to form approximately cylindrical face with three curvature radiuses, so that the pressurizing force of the vibration control wires can largely be applied especially to the points where the AG tape is easily vibrated.
In recent years, there has been an increase in so-called flat color CRTs, in which the glass panel surface is substantially flat. FIG. 7 shows a partial perspective cross-sectional view of a glass panel 700 for use in a flat CRT. In this drawing, the reference numeral 701 is a glass panel surface (outside of the color CRT), and 702 is the glass panel inner surface (inside of the color CRT). As shown in this drawing, even if the glass panel surface 701 is substantially flat, the thickness of the glass panel cross-section is thicker toward the periphery than at the center, and has the shape of a concave lens that forms a part of a cylinder. The first reason for this is that, if the glass thickness at the periphery were to be the same as at the center, the withstanding pressure of the glass bulb would become low, and because there is a great danger of implosion in a color CRT. The second reason is that, if the glass thickness at the periphery is the same as that at the center, the image will appear to have an unnatural dimpling effect. To prevent implosion and impart an appearance of flatness to the image, the glass thickness is made greater toward the periphery, with respect to a reference thickness at the center of the glass panel cross-section. The amount of increase in glass thickness at the periphery with respect to the center is the wedge amount W, as shown in the drawing. The wedge amount W has an appropriate value, which is dependent upon the size of the color CRT. For example, for a 19-inch color CRT, the proper wedge amount is 3 mm.
Because as noted above the glass panel inner surface 702 has the shape of a concave lens that forms a part of a cylinder, a shadow mask of the past was accordingly made as a cylinder type with a constant radius of curvature that is substantially congruent with the glass panel inner surface 702.
FIG. 8 is a perspective view of a shadow mask structure 800 of the past. In this drawing, the reference numeral 801 is a mask frame, 802 is a shadow mask (only the periphery of which is shown, to make the inside of the mask frame 801 more visible), and 802A is a shadow mask welding point. The shadow mask 802 is tensioned via the shadow mask welding point 802A at two sides of the mask frame 801.
FIG. 9 is a cross-sectional view of the shadow mask 800 of the past, in the direction of X-Xxe2x80x2. As shown in this drawing, the shadow mask 800 of the past has a constant radius of curvature R0 everywhere, and is cylindrical in shape. This radius of curvature R0 is, for example, approximately 4050 mm for a 19-inch color CRT.
The above-described shadow mask 800 of the past, however, has the following problems. When manufacturing a color CRT, the shadow mask 802 is tensioned onto the mask frame 801 at room temperature (approximately 25xc2x0 C.). For this reason, the shadow mask 802 maintains a normal tensioned condition, with no deformation, at room temperature. That is, the shadow mask 802 maintains a non-deformed and normal tensioned condition when the color CRT is not being used.
However, when the color CRT is in use, because an electron beam collides with the shadow mask 802, the shadow mask structure 800 rises to a temperature of approximately 60xc2x0 C. Because the shadow mask 802 is made of Invar (36% nickel iron alloy), which has a low coefficient of thermal expansion, there is almost no thermal expansion, the coefficient of thermal expansion of Invar being approximately 1.2 ppm/K at room temperature. Because the mask frame 801, however, is made of 13 chromium stainless steel, which has a coefficient of thermal expansion approximately 10 times that of Invar, it exhibits considerable thermal expansion. Because of the difference in thermal expansion between the shadow mask 802 and the mask frame 801, the shadow mask 802 is subjected to stress.
FIG. 10 is a plan view of the shadow mask 802 of a shadow mask structure 800 of the past, seen from the direction A. In this drawing, F indicates the distribution of stress on a side of he shadow mask 802 attributed to the difference in coefficients of thermal expansion between the shadow mask 802 and the mask frame 801. Because the shadow mask 802 is free in the lateral direction, stress is not applied to the left and right sides thereof. In contrast to this, because the top and bottom sides of the shadow mask 802 are welded to the mask frame 801, because of the thermal expansion of the short side of the mask frame 801, a stress F is applied to the as shown in the drawing, this being small at the center of the side and large at the periphery of the side. Because of the non-uniform distribution of this stress F, a stress is developed in the shadow mask 802 perpendicular to the stress F, thereby pulling the shadow mask 802 downward, resulting in the deformation of the shadow mask described below.
FIG. 11 shows a plan view of the shadow mask 802 of the shadow mask structure 800 of the past from the direction A of FIG. 8, and the X-Xxe2x80x2 and Y-Yxe2x80x2 cross-sectional views thereof. In these cross-sectional views, the condition shown is that in which the shadow mask 802 is at 25xc2x0 C. (broken line) and at 60xc2x0 C. (solid line). As shown in this drawing, when the temperature of the shadow mask structure rises, the shadow mask 802 is deformed by pulling in a direction that moves it away from the glass panel 700 of the color CRT. The amount of deformation is greatest near the center of the shadow mask 802 and, by experiments performed by the inventors, this was found to be approximately 100 xcexcm near the center, as shown in the drawing. As is known, when the distance between the shadow mask and the color panel 700 changes, landing error occurs in the electron beam. When this occurs, the change in distance between the shadow mask 802 and the glass panel 700 is greatest near the center of the shadow mask 802, and because the electron beam is incident substantially perpendicularly to the glass panel 700 at in the center region, landing error tends not to occur. At the periphery of the shadow mask 802, because the change in distance between the shadow mask 802 and the glass panel 700 is small, the landing error is small. As a result, in the region 802B indicated by hatching in FIG. 12 (plan view of the shadow mask 802), that is, in a range of ⅙ to {fraction (2/6)} from both edges of the shadow mask 802, the landing error is the greatest. An experiment by the inventors indicated that, for a deformation of approximately 100 xcexcm at the center region of the shadow mask 802, the landing error in the region 802B is approximately 20 xcexcm. With an electron beam landing error of 20 xcexcm, there is a clearly perceivable decrease in color purity.
That is, a shadow mask structure of the past, which had a cylindrical shadow mask with a constant radius of curvature, there was the problem of a decrease of color purity when the color CRT was used.
Accordingly, it is an object of the present invention to provide a shadow mask structure that, by properly establishing the radius of curvature of the shadow mask, has a small amount of shadow mask deformation and a small amount of landing error, even if the temperature of the shadow mask rises when the color CRT is used.
It is a further object of the present invention to provide a color CRT using the above-noted shadow mask, which features superior color purity, a natural flat appearance, and sufficient withstanding pressure.
In order to achieve the above-noted objects, a shadow mask structure according to the present invention features radii of curvature at the center part of the screen and at the peripheral part of the screen that are appropriately established as different values.
Specifically, a first aspect of the present invention is a shadow mask structure formed by a mask frame having a substantially rectangular outer frame and a shadow mask that is substantially rectangular and that is tensioned on a pair of sides of the mask frame, wherein the part of the mask frame onto which the shadow mask is tensioned is formed by a combination of a plurality of arcs, the radii of curvature of the plurality of arcs becoming successively smaller with movement from the mask frame center to the mask frame periphery, the cross-section of the shadow mask that is parallel to the shadow mask tensioned part having substantially the same shape as the shadow mask tensioned part of the mask frame.
In a second aspect of the present invention the shadow mask tensioned part of the mask frame is a combination of arcs that has two radii of curvature, wherein approximately {fraction (4/6)} at the center part of the mask frame has the larger of the two radii of curvature, and the left and right ⅙ have the smaller of the two radii of curvature.
In a third aspect of the present invention, the larger radius of curvature of the arc is approximately 4500 mm to 6000 mm, and the smaller radius of curvature of the arc is approximately 1000 mm to 2000 mm.
In a fourth aspect of the present invention, the material of the shadow mask tensioned part of the mask frame is Invar, with the material of the other parts of the mask frame being 13 chromium stainless, the shadow mask material being Invar.
A fifth aspect of the present invention is a shadow mask having a mask frame having a substantially rectangular outer frame and a shadow mask that is substantially rectangular, this shadow mask structure having a pair of mask supporting elements on a pair of longer sides of the mask frame, a pair of long sides of the shadow mask being tensioned to the mask supporting elements, and the shadow mask tensioned part of the mask supporting elements being formed by a plurality of arcs, the radii of curvature of the plurality of arcs becoming successively smaller with movement from the center of the mask supporting elements toward the periphery thereof, a cross-section of the shadow mask that is parallel to the mask supporting elements having a shape that is substantially the same as the shadow mask tensioned part of the mask supporting elements.
In a sixth aspect of the present invention, the shadow mask tensioned part of the mask supporting elements is a combination of arcs that has two radii of curvature, where substantially {fraction (4/6)} of the center part of the mask supporting elements is an arc having the larger of the radii of curvature and the substantially ⅙ of the shadow mask supporting elements to the right and left sides have the smaller of the two radii of curvature.
In a seventh aspect of the present invention, the larger radius of curvature of the arc is approximately 4500 mm to 6000 mm, and the smaller radius of curvature of the arc is approximately 1000 mm to 2000 mm. In an eighth aspect of the present invention, the material of the mask frame is 13 chromium stainless steel, and the material of the mask supporting elements and the shadow mask is Invar.
In a ninth aspect of the present invention, the shape of the hole in the shadow mask through which the electron beam passes is substantially rectangular.
A tenth aspect of the present invention is a color CRT making use of a shadow mask structure according to any one of the first through the ninth aspects of the present invention.